Amplifier circuit having feedback connection for reducing distortion caused by the non-linear characteristic of its output transformer



Dec. 10, 1968 G. J. OVERTVELD 3,416,091

7 AMPLIFIER CIRCUIT HAVING FEEDBACK CONNECTION FOR REDUCING DISTORTIONCAUSED BY THE NON-LINEAR CHARACTERISTIC OF ITS OUTPUT TRANSFORMER FiledMarch 5, 1965 PRIOR ART 3 1| INVENTOR G. J. OVERTVELD United StatesPatent 3,416,091 AMPLIFIER CIRCUIT HAVING FEEDBACK CONNECTION FORREDUCING DISTOR- TION CAUSED BY THE NON-LINEAR CHARACTERISTIC OF ITSOUTPUT TRANSFORMER Gilles Jozias Overtveld, Ottawa, Ontario, Canada,assignor to Northern Electric Company Limited, Montreal, Quebec, CanadaFiled Mar. 5, 1965, Ser. No. 437,508 Claims. (Cl. 330-) ABSTRACT OF THEDISCLOSURE This invention relates to feedback pair amplifiers whichrequire the use of an output transformer for connecting the amplifiersto load circuits. Because such amplifiers have very low distortion, theyare much affected by the distortion caused by the output transformerwhich is normally used to couple such transformer to the load circuit.In accordance with the present invention, the distortion caused by thenon-linear characteristic of the transformer itself is lowered byinserting the transformer in the feedback loop of the amplifier. Moreparticularly, the primary winding of the output transformer is providedwith taps and such taps are connected in the feedback loop of theamplifier to compensate for the distortion caused by the transformer.

This invention relates to the field of audio amplifiers used intelephone, broadcasting and public address systems and more particularlyto an audio amplifier having low output distortion and having animproved output impedance characteristic.

Audio amplifier components behave linearly only for small signals. Forlarge voltage and current excursions other signals than the drivinginput signal will be generated and appear in the output as distortion.To obtain output power with a specified distortion two avenues are open:the first one being to make the excursions small enough over a linearportion of the component characteristics; and the second one being theapplication of negative feedback.

The first method is uneconomical because components such as tubes,transistors, transformers are linear over a small portion of theircharacteristics only. The second method is well known and widelyapplied. It can be shown that the gain of an audio amplifier usingnegative feedback is:

e =input voltage of the amplifier. e =output voltage of the amplifier.A=gain of the amplifier without feedback. B=amount of negative feedback.

The reason for applying negative feedback is mainly to decrease thedistortion because the distorted signal is fed back into the amplifierin such a way as to tend to cancel the distortion originally generated.

A number of circuits have been designed that use the negative feedbackprinciple. Among them are the so-called complementary symmetry and theseries push-pull circuits. A more recent circuit is the so-calledfeedback pair a description of which may be found in the Marconi Reviewof the 1st Quarter 1962.

These as well as other prior art amplifier circuits require an outputtransformer for connecting the amplifier to the telephone circuits,broadcasting circuits or other 3,415,091 Patented Dec. 10, 1968 ice . 2output loads. The insertion of the transformer however effects thedistortion, and the output impedance of the amplifier. Distortion iscaused by the non-linearities in the B-H curve of the outputtransformer. The output impedance increases with frequency due to theleakage reactance of the primary and secondary windings.

Considering in particular the prior art feedback pair amplifier, thedrawbacks of that circuit have been overcome in accordance with theinvention by providing an amplifier including the output transformer inthe feedback loop of the amplifier. More particularly applicant hasdiscovered that by substituting the resistances of the output circuit ofthe so-called feedback pair mentioned previously by the primary of theoutput transformer, an amplifier is obtained having improved outputcharacteristics over the prior art amplifiers with regard to distortionand output impedance.

The invention will now be described with reference to the followingdrawings in which:

FIG. 1 illustrates the so-called feedback pair amplifier;

FIG. 2 illustrates the feedback pair amplifier modified in accordancewith the invention; and

FIG. 3 is a push pull circuit embodying two feedback pairs modified inaccordance with the invention.

With reference to FIG. 1, there is shown a feedback pair comprising PNPtransistor Q and NPN transistor Q The input voltage is applied to thebase of transistor Q and the output voltage is taken from acrossresistors r and r in the collector circuit of transistor Q The base oftransistor Q is connected to the junction point between resistors 1' andr.;. The collector of transistor Q is connected to the base oftransistor Q and to a source of D.C. potential -V through a resistor rThe emitter of transistor Q, is connected to a point between resistors1' and r The collector of transistor Q is connected to ground throughresistors 1' and r The emitter of transistor Q is connected to thesource of DC. potential V through a reference diode Z.

Assuming now that Q and Q are a PNP and a NPN transistor respectivelyhaving a current gain B and B 2= 1 1 i =i if r input impedance of Q thenr2 1 2 1 r 1 2 1+ 1 1 If e aV (the effect of Q junction neglected) 1= r1=( 1 2 1+ 1 1) 1 Neglecting B 1" because B l,

Equation 6 shows up a very useful property of this circuit which is highinput impedance.

Considering now the gain of the circuit,

substituting i from (5) n-i-r ==volta e am 61 g g 3 and therefore verylinear provided that r is much larger than the input impedance of Q Sofar signals only have been considered. Proceeding now with the D.C.analysis of the circuit, the DC. current in Q is V V [Q1 BBT C1 where V=collector voltage of transistor Q Now for a silicon transistor the baseemitter voltage of transistor Q is about 0.7 v. and so (V V would be 0.7v. in the absence of the reference diode. (For germanium the value is0.2 v.). Consequently unless the value of r is small a very smallcurrent will flow in Q and this is likely to be inadequate. If r issmall then To overcome this problem I proposed in accordance with theinvention to modify the circuit such as shown in tion at maximum outputpower and bandwith.

The output characteristics of the amplifier circuit made PE RFO RMANOEDisclosed circuit Compared Circuit )istortion Po=18 dbm,

percent:

From the above table it can be deduced that:

) The distortion is less than the comparison circuit, )The outputimpedance varies less with frequency than the comparison circuit.

[t is to be understood that the invention also contemplates aninterchange of PNP and NPN transistors in FIGURE 2 of the drawings. Apush pull configuration URE 2. The input voltage is applied to the baseof transistors Q and Q The tap connections to the emitter electrodes oftransistors Q and Q. A source of DC.

crease the distortion produced by the output transformer due to itsnon-linear characteristic.

2. An amplifier circuit comprising:

(a) a first and a second transistor of opposite conductivity, eachtransistor having base, emitter and collector electrodes;

(b) means for feeding a signal to the base of the first transistor;

(c) means for connecting the collector of the first transistor to thebase electrode of the second transistor;

(d) an output transformer having a primary winding connected to thecollector electrode of said second transistor and a secondary windingfor connection its non-linear characteristic. 3. An amplifier circuit asdefined in claim 2 wherein whereby most of the signal current flows intothe base of the second transistor.

4. An amplifier circuit as defined in claim 2 wherein the emittercircuit of the second transistor includes a DC. potential source of apolarity such as to increase the DC. current in the first transistor upto a predetermined value.

5. An amplifier circuit as defined in claim 4 wherein the DC. potentialsource is a reverse biased diode.

sistor is a NPN transistor.

7. A push pull amplifier circuit comprising:

(a) two pairs of transistors of opposite conductivity, each transistorhaving base, emitter and collector electrodes;

(b) means for feeding a signal between the base electrodes of the firsttransistor of the pairs;

(0) means for connecting the collector electrode of (d) an outputtransformer having a primary winding connected between the collectorelectrodes of the second transistor 0f the pairs and a secondary windingfor connection to a load circuit; and

(e) means interconnecting a tap point on the primary winding and theemitter electrode of the first transistor for feeding back a portion ofthe voltage developed across the primary winding of said transformer tothe emitter electrode of the first transistor of each pair to decreasethe distortion produced by the output transformer due to its nonlinearcharacteristic.

8. A push-pull amplifier circuit as defined in claim 7 wherein thecollector circuit of the first transistor of each pair includes aresistance element having an impedance which is much higher than theinput impedance of the second transistor whereby most of the signalcurrent flows into the base of the second transistor.

9. A push-pull amplifier circuit as defined in claim 7 wherein theemitter circuit of the second transistor of each pair is common andincludes a DC. potential source of a polarity such as to increase theDC. current in the first transistor of each pair up to a predeterminedvalue. 10. A push-pull amplifier circuit as defined in claim 9 whereinthe DC. potential source is a reverse biased diode.

References Cited 7 UNITED STATES PATENTS 3,105,198 9/1963 Higgin Bo tham330l7 XR 3,166,719 1/1965 Wiencek 330-47 3,312,833 4/1967 Durrett 330-15X FOREIGN PATENTS 849,316 9/1960 England.

' JOHN KOMINSKI, Primary Examiner.

L. J DAHL, Assistant Examiner.

US. Cl. X.R.

